For example, a power supply device for electric power tool described in Patent Document 1 mentioned below is composed of an adapter to be electrically connected to an electric power tool, a rechargeable battery which supplies an electric power to the electric power tool, and so on.
In the invention described in Patent Document 1, in a case where an electric power stored in the rechargeable battery decreases and thus, the rechargeable battery reaches a deeply-discharged state, since an electric power sufficient to operate the electric power tool cannot be obtained from the rechargeable battery, it becomes necessary to charge the rechargeable battery.